The present invention pertains to a method and a system for reducing at least one frequency component of a periodic pulsation in at least one electrical parameter at an output of a synchronous generator powered by a pulsating drive momentum of a drive engine, where the fundamental frequency of the pulsation with said component to be reduced deviates from the natural frequency of said synchronous generator.
In addition, the present invention pertains to the use of the method and system for large, Diesel engine-driven synchronous generators.
When a synchronous generator is driven with a momentum or torque which fluctuates in time, this affects the electrical parameters at the output, that is, on the stator side of the generator. These fluctuations of momentum occur especially when large Diesel engines are used to drive synchronous generators and when wind turbines are used as power sources. The generators are driven in these cases at relatively low speeds of 60-120 rpm.
The pulsations in the momentum driving the synchronous generator are usually not sinusoidal, so that pulsation components of higher harmonic frequencies with respect to the fundamental frequency of the pulsation are produced; that is, the frequency spectrum of the pulsation is discrete, with several frequency components or spectral lines, among which there is also the component accounted for on the fundamental frequency.
Depending on the frequency behavior of the synchronous generator, its drive shaft, and its output-side network, one or more of these components are amplified to a particular degree as a result of the resonance behavior of the partial stages mentioned. Thus, in turn, a pulsation with various frequency components usually appears on the output side of the generator, and certain of these frequency components appear in amplified form. When we speak in the following of a frequency component of the pulsation to be reduced on the output side of the synchronous generator, the fundamental frequency of which differs from the synchronous generator's own frequency, that is the system frequency, we mean the spectral line of the pulsation which appears on the output side with such a large amplitude that the disturbance thus caused should be reduced. The frequency of the most highly troublesome component can usually change over the course of time, e.g., when the resonance behavior of the partial stages mentioned above changes, as in the case of changes in load on the network side.
Synchronous generators are usually equipped with a so-called P/f control system which is a system for active power control by torque adjustment, by means of which the mean value of the active power transmitted to the network is kept constant. This is done by adjusting the torque of the drive engine.
Fluctuations in the active power output P cannot be corrected quickly, however; they are, in fact, corrected much more slowly than would be necessary to eliminate the pulsation component to be reduced as mentioned above, which has a frequency in the range of a few Hz. When a synchronous generator of this type is used as a power plant generator on a rigid network, a so-called Q/U controller which is a system for output voltage (i.e., reactive power) control by exitation adjustment of the generator is also usually provided. On the output side of the generator, the reactive power Q is measured, and the mean value of the reactive power is kept constant by adjusting the excitation of the generator, that is, by adjusting the excitation voltage. This form of control can also be much too slow in the sense described above.
In a rigid network, the pulsations in the momentum of the drive engine mentioned above bring about pulsations of active power components with amplitudes in the area of about 5-20% of the mean active power at pulsation component frequencies of a few Hz.
When a power plant system of this type is operated in an isolated mode, a U-control system is usually installed, which latter system taps the voltage being sent to the network and keeps it constant by controlling the excitation of the synchronous generator. In this case, the pulsations in the momentum of the drive engine indicated bring about voltage component amplitudes in the area of 10 Hz on the order of 0.5% of the mean voltage value.
The pulsation components mentioned cause disturbing effects on the load side both in rigid networks and in isolated systems. When the system is part of a rigid network, these pulsations act on the other power plants connected to the same network, which attempt to smooth out these pulsations; in an isolated system, however, interfering effects occur in spite of the relatively small amplitudes of the component mentioned, i.e., precisely in the range of 10 Hz, such as disturbances in light sources connected in the network, in that this frequency is situated precisely in the area of the greatest sensitivity of the eye, which is also in the area of 10 Hz. In this range, the eye perceives even very slight variations in the amplitude of light sources. The effects of the moment pulsations indicated can, as mentioned, appear in even more emphatic form when the fundamental frequency of the momentum pulsations or the frequency of a harmonic component is situated at a resonance point of a synchronous generator connected to a network or at a resonance point of the drive shaft between the drive engine and the synchronous generator.
It is known, for example, from U.S. Pat. Nos. 4,080,559; 4,463,306; and 4,413,223; from G. R. Phillips, "A microprocessor-based engine/generator control system", IEEE, 1980, IECI Proceedings, "Applications of Mini- and Microcomputers", Philadelphia, Mar. 17-20, 1980, IEEE, U.S., pp. 377-380; and from E. N. Hinrichsen, "Controls for variable-pitch wind turbine generators", IEEE Transactions on Power Apparatus and Systems, Vol. PAS-103, No. 4, April 1984, IEEE, New York, U.S., pp. 886-892, that system stabilizers can be provided for synchronous generators powered by drive engines, by means of which the overall system is stabilized in principle by shifting appropriately to the left the pole positions of the system transfer function.
As known from U.S. Pat. No. 4,080,559, however, when stabilizers of this type are installed, the problem is that low natural frequencies of the mechanical coupling are excited, and that thus the entire system together with the stabilizers acts as a positively fed back system at these low frequencies. In the U.S. patent last mentioned above, this problem is solved by interrupting the action of the stabilizer at these natural frequencies. The feed-back control circuit is made ineffective at these natural frequencies, and the system thus operates without feed-back control at these frequencies. The result of this measure is that the pulsations brought about by the stage consisting of the drive engine and the generator is no longer excited to oscillate at these natural mechanical frequencies; instead, they appear undamped at a greater or lesser intensity on the output side in accordance with the fact that the feed-back system is now open at these frequencies. Whereas in this way, especially as indicated in this publication and generally in conjunction with the installation of stabilizers to stabilize the entire system, the goal is to avoid exciting pulsations, which e.g. arise on such natural mechanical frequencies, the present invention is based in principle on not simply leaving pulsations with such low fundamental frequencies undamped in their unaffected form but rather on reducing them. Thus the goal of the present invention is to reduce the pulsation components indicated on the output side of the synchronous generator, thereby keeping stability of the overall system, in terms of automatic feed-back system theory.